Outer hair cells (OHC) are required for normal mammalian hearing. They amplify sound vibrations in the inner ear through their ability to convert electrical to mechanical energy. Interfering with the force generating mechanism results in hearing loss. The mechanism responsible for this force production is unknown but it resides in the OHC's lateral wall. The lateral wall is an elegant, three layered, composite nanostructure that is as structurally conspicuous in the OHC as the force production. The long-term goal of this proposal is to understand the mechanism by which the nanoscale mechanical anatomy of the lateral wall facilitates OHC electomechanical force production, particularly at acoustic frequencies. Experimental and theoretical approaches will determine how the organization of lateral wall directs mechanical force along the OHC's axis and compensates for viscous damping. We will investigate the characteristics of two plausible molecular mechanisms, one driven by in-plane conformational changes of a motor protein and the other by out-of-plane flexoelectric bending. Contemporary electrophysiological and advanced optical techniques will be used. Specific Aim 1 will test the hypothesis that the elastic properties of the lateral wall vary with membrane potential. Local cell wall strains will be measured with optical tweezers and under conditions of micropipet aspiration and osmotic challenge at different holding potentials. Specific aim 2 will establish the magnitude of out-of-plane bending of the plasma membrane in response to active and passive length changes of the OHC using polarized evanescent illumination. Specific aim 3 is to establish the role of the lateral wall cytoskeleton in maintaining cell shape and transmitting active forces. The cell's electromotile response during damage and recovery of the cytoskeleton will allow a determination of its active force transmission. Specific aim 4 will result in a complete nanoscale model of OHC active force production. It will include full dynamic description of the trilaminar lateral wall mechanical anatomy. The model will allow us to predict force-deformation relationships for the OHC at acoustic frequencies and it will meet all the criteria for the mechanism of OHC force production.